1. Field of the Invention
The present invention relates to a method for developing a shallow trench isolation (hereinafter, called STI) in a semiconductor memory device, and more particularly, to an improved method for forming an upper corner of a trench used to achieve STI in a semiconductor memory device, which is rounded.
2. Description of the Prior Art
Referring to FIGS. 1A through 1H, description of the conventional STI method will now be given in detail.
First, as shown in FIG. 1A, a first oxide film 12 is formed on a semiconductor substrate 11, and a nitride film 13 is formed on the first oxide film 12. Then, as shown in FIG. 2B, a photoresist pattern 14 is formed on the nitride film 13, and the first oxide film 12 and the nitride film 13 are patterned by using the photoresist pattern 14.
Next, as shown in FIG. 1C, the photoresist pattern 14 is removed, and then a trench 15 is formed in the semiconductor substrate 11 by etching the substrate 11 using the patterned nitride film 13 as a hardmask.
Then, as shown in FIG. 1D, by performing a light oxidation, a second oxide film 16 is formed on the inner surface of the trench 15, and, as shown in FIG. 1E, a third oxide film 17 is formed on the entire resultant upper surface by using a high density plasma (HDP) deposition. Here, the trench is filled with the third oxide film 17.
As shown in FIG. 1F, the entire resultant surface of the construction is polished using an apparatus (not illustrated) for a chemical mechanical polishing (CMP). Then, as shown in FIG. 1G, the residual nitride film 13 is removed by etching, and finally, as shown in FIG. 1H, the oxide films 12, 16, 17 are removed by etching, except the portions of the second and third oxide films 16, 17 filled in the trench 15, whereby the conventional STI process of the semiconductor memory device is completed.
As shown in FIG. 1H, the semiconductor structure formed by the conventional STI method has an upper corner portion of 15a of the trench formed in a rectangular construction. Thus, when the completed semiconductor memory device is operated, an electric field is concentrated in the corner portion 15a, resulting in a hump phenomenon.